Electrographic marking upon an image recording medium comprises a two-stage process. First, air ions are created and charged ions of a given sign (usually negative) are deposited at selected image pixel locations to form an electrostatic charge image on a recording medium. Then, the electrostatic charge image is made visible by "toning", which usually involves the passing of the recording medium bearing the latent (non-visible) image, into contact with a liquid solution containing positively charged dye particles in a colloidal suspension. The dye particles will be attracted to the negative charge pattern and the density of the dyed image will be proportional to the potential or charge on the medium.
Two types of recording media that are in common usage are paper and film. The paper is usually treated to make its bulk conductive and a dielectric layer of about 0.5 mil thick is coated upon its image bearing side. In its dielectric film form, a substrate such as Mylar.RTM., has a very thin conductive layer and an overcoat dielectric layer coated upon its image bearing side. Conductive side stripes pass through the dielectric layer to the conductive layer provide electrical paths to the conductive layer. In the case of paper, the potential established in the conductive layer is obtained by a combination of resistive and capacitive coupling, and in the case of film, the potential established in the conductive layer is obtained by capacitive coupling.
Conventionally, as illustrated in FIG. 1, an electrostatic image is formed upon a recording medium 10 having a thin surface dielectric layer 12 coated upon a conductive paper base material 14. The recording medium is passed between a recording head 16 and an array of complementary electrodes 18. The recording head includes an array of recording stylus electrodes 20, divided into groups, embedded in a dielectric supporting member 22. In the drawing, the complementary electrodes are in the form of backplates which conform to the contour of the recording medium for intimate contact therewith. Alternatively, they may straddle the stylus electrodes, on the same side of the recording medium. Throughout this document the term backplate will be used interchangeably with complementary electrode and it should be understood that frontplate electrodes are contemplated as well.
When the potential difference between the stylus electrodes and the recording medium conductive layer rises enough to cause the voltage in the air gap to exceed the breakdown threshold of the air, the air gap becomes ionized and air ions of the opposite sign to the potential of the conductive layer are attached to the surface of the dielectric layer. As the dielectric surface charges up, there is a corresponding drop in voltage across the gap, so that when the voltage across the gap drops below the maintenance voltage of the discharge, the discharge extinguishes, leaving the dielectric surface charged. The discharge potential is established by applying a voltage of a first polarity, e.g. on the order of -300 volts, to the stylus electrodes contemporaneously with the application of a substantially equal voltage of the opposite polarity, e.g. +300 volts, to the complementary electrodes. This causes the electrical discharge, imposing a localized negative charge to the surface of the dielectric layer 12 of the recording medium.
Typical electrographic plotters range in width from 11 inches to 44 inches, and in some cases even as wide as 72 inches, with the writing head stylus array extending across the width. Since images are usually formed at resolutions of 200 to 400 dots per inch, there are from 2000 to over 17,000 styli in a single array. Because of this very large number of styli it is not yet economically attractive to use one driver or switch per stylus. For this reason, a multiplexing arrangement is commonly used in conjunction with the discharge method described above wherein one part of the total voltage, necessary for electrographic writing, is imposed upon a stylus group and the remaining part of the necessary voltage is imposed upon its complementary electrode. The styli in the writing head array are divided into a number of stylus electrode groups (each group being about 0.5 inch to 1.5 inches in length) so that each may consist of several hundred styli.
In order to reduce the number of drivers needed, since one driver can be used for many styli, like styli in each group of stylus electrodes are wired in parallel so that they carry the same information. Then, in order to cause a selected stylus group to write, its complementary electrode is selectively pulsed. It is well known that any practical number of groups is possible. In FIG. 2 there is illustrated the conventional form for the multiplexed addressing of two alternating stylus electrode group (referred to as "A"s and "B"s). The recording medium 10 passes between the stylus groups 20 and the backplates 18. Each commonly numbered stylus in each "A"-stylus group is wired in parallel with each like numbered stylus in every other "A"-stylus group. Similarly, all "B"-stylus groups are wired in parallel. Each of the stylus groups is the same length as the complementary electrode and they are offset with respect to one another so that two adjacent complementary electrodes are needed to cause a writing discharge from one stylus group. By having two complementary electrodes generally centered relative to a given selected stylus group, the voltage across the recording medium can be expected to be uniform. Although the leading and trailing alternate stylus groups adjacent to the given selected stylus group are also influenced by an overlapping portion of the selected complementary electrodes they will not write because they are not addressed and enabled.
A related patent application filed contemporaneously herewith and fully incorporated by reference herein, identified by U.S. Ser. No. 07/532,467 filed on May 30, 1990 entitled "Electrographic Marking With Modified Addressing To Eliminate Striations" relates to the formation of objectionable striations introduced by pulsing of the complementary electrodes, and to a method for eliminating them by changing the stylus electrode group firing sequence.
We have found that the pulsing of the stylus electrode groups themselves are a cause of other striations extending in the process direction which appear as enhanced, or darker, areas at the electrode group boundaries. The styli themselves can be brought of as small electrodes that are capacitively coupled to the conductive layer of the recording medium, in a manner comparable to the effect of the complementary electrodes upon the recording medium. Although the styli are small they are closer (in the case of film) to the conductive layer than are the backplate electrodes and therefore have more capacitance per unit area with respect to the conductive layer than do the backplates. When the styli are pulsed, they also pulse the conductive layer which must dampen out that potential perturbation before the full voltage is available for writing.
For simplicity, we discuss the model of writing all black, wherein all the styli in a set are pulsed ON to a writing potential. The potential on the recording medium conductive layer adjacent to the styli is pulsed negative when the styli are pulsed negative. At both edges of a stylus electrode set, the potential of the conductive layer relaxes away faster than the potential on the conductive layer in the center of an electrode set so that the potential difference between the styli and the medium in those areas is greater. As a result, the edge areas of the stylus electrode sets accumulate more charge and write darker than the center of the sets. Therefore, as the plotter writes, the aligned edges result in dark stripes extending in the direction of movement of the recording medium (i.e. the process direction).
It is the primary object of the present invention to improve the uniform appearance of writing by preventing the alignment of stylus electrode set edges which can be accomplished by dithering the sets in subsequent line scans.
It is another object of the present invention to selectively determine the lateral location of stylus electrode sets within a scan line with a zero mask.
It is yet another object of the present invention to increase the lateral distances that the stylus electrode sets may be shifted by reducing the length of the backplate electrodes.